The detection of electromagnetic radiation is one of the most important sensing tasks in science, technology and consumer electronics. Silicon is the fundamental semiconductor of microelectronics, and it is very well suited for the fabrication of highly sensitive photosensors—from point detectors to multi-megapixel image sensors—covering the wide spectral domain from the soft X-ray region to the near infrared. This corresponds to a sensitive wavelength range from about 1 nm up to the cutoff-wavelength of silicon of about 1100 nm. Due to the growing importance of infrared radiation for diagnostic and non-contact chemical fingerprinting purposes, there is also a rapidly growing need for photosensors that are sensitive in the infrared spectral domain with a wavelength above 1 micrometer, where conventional silicon photosensors are not sensitive anymore.
A widely-used method for the detection of infrared radiation is the use of pyroelectric materials, capable of spontaneously changing their electric polarization as a function of temperature, as described for example by J. Fraden in “Handbook of Modem Sensors”, 3rd edition, Springer 2004. This polarization change can be detected as a current or voltage change, employing known electronic measurement circuits. Because of the difficulty of processing most pyroelectric materials, only limited numbers of photosensing elements (pixels) are fabricated on a single detector device, typically between one and several hundred pixels.
In a thermopile, larger numbers of infrared-sensitive pixels are manufactured with thermoelectric material systems, as described for example by J. Fraden in “Handbook of Modem Sensors”, 3rd edition, Springer 2004. They consist of a combination of two different types of semiconductor materials, a so-called thermocouple. Due to the Seebeck effect, such a thermocouple produces a voltage as a function of a temperature difference across the device. A thermopile sensor is composed of several hundred to several thousand thermocouple pixels on the same device.
Even larger numbers of infrared pixels—up to several 100,000 pixels on a device—can be fabricated with microbolometer arrays, as described for example by J. Fraden in “Handbook of Modem Sensors”, 3rd edition, Springer 2004. Each pixel consists of a thermally isolated heat absorber on top of a conducting material that shows a large resistance change as a function of temperature. Microbolometer sensors can be operated without cooling. However, because their working principle depends on device heating, the sensitivity of microbolometers is quite restricted, and it is impossible to come close to single-photon sensitivity.
This limitation can be overcome with infrared sensing devices based on the photoelectric effect, as described for example by B. E. A. Saleh and M. C. Teich, “Fundamentals of Photonics”, 2nd edition, John Wiley and Sons, 2007. In a first type of photoelectric devices, the external photoelectric effect in metals and semiconductors in a vacuum is exploited. If an incident photon of sufficient energy is absorbed by an electron in the photoelectric material, this excited electron can overcome the attractive force of the material, so that the electron can leave the material and enter the vacuum space. In the vacuum, this liberated electron can be supplied with additional energy, often by accelerating it in a high-voltage electric field, so that each individual electron can be detected reliably.
In order to simplify manufacturing of an infrared sensor and to lower the cost of production, one tries to avoid employing a vacuum. This can be accomplished with the internal photoeffect, exhibited by semiconducting material systems. These materials show a bandgap structure in their energy diagram, with a fully occupied valence band and a fully empty conduction band at zero absolute temperature. If the energy of an incident photon is larger than the bandgap—the energy difference between conduction band and valence band—then the incident photon can be absorbed by the semiconducting material and can create a pair of mobile charges, an electron in the conduction band and a hole in the valence band. In this way, incident radiation modifies the electrical conduction properties in the semiconducting material, which can be sensed with electrical circuits. In photoconductive sensors, the change of effective resistance is measured as a function of the intensity of the incident radiation. In photovoltaic sensors, the photogenerated charge pairs move in an electric field, creating an electrostatic potential change across the device as a function of the intensity of the incident radiation. The most sensitive photosensors that are based on the internal photoeffect consist of depleted semiconductor regions, created either with reverse-biased photodiodes, as used for example in CMOS image sensors, or with MOS (Metal Oxide Semiconductor) structures, as used for example in Charge Coupled Device (CCD) image sensors or photogate image sensors. In these sensitive photosensors, the devices are biased to a certain reverse potential, and then they are left electrically floating. The photogenerated charge carriers reduce the voltage across the device in proportion to the intensity of the incident radiation. This voltage change can be electrically detected with a room-temperature readout noise corresponding to less than one electron r.m.s., as described for example by Ch. Lotto and P. Seitz in European Patent No. 8,119,972 B2, “Solid state image sensing device having a low pass filter for limiting signal frequencies passing to the output node of an inverting amplifier”.
All of these sensitive radiation detectors employing the external or internal photoeffect have in common that they cannot detect incident photons whose energy is too low to either overcome the effective work function in the case of the external photoeffect or to create mobile charge pairs over the bandgap in the case of the internal photoeffect. As a consequence, these sensitive photodetectors have a so-called cutoff wavelength λC above which they are not sensitive any more. The cutoff wavelength λC is inversely proportional to the minimum energy Emin, required to create mobile charge carriers due to the photoeffect, λC=h×c/Emin, with Planck's constant h and the vacuum speed of light c. This implies that the photoelectric effect is unsuitable for the detection of electromagnetic radiation in the infrared spectral range with its particularly long wavelengths.
This limitation of absent infrared sensitivity can be overcome with a semiconductor device according to the HIP (homojunction internal photoemission) principle, as described for example by A. G. U. Perera et al. in “Homojunction interal photoemission far-infrared detectors: Photoresponse performance analysis”, J. Appl. Phys. Vol. 77, pp. 915-924, 1995. A HIP detector consists of a vertical arrangement of a highly doped semiconductor region at the surface of the device, followed by a lightly doped (or intrinsic) region. In the highly doped region, a large number of free charge carriers are present, and these can interact with the incoming electromagnetic radiation through free carrier absorption (FCA). A free charge carrier can absorb the energy of an incident photon, resulting in a photoexcited charge carrier. These photoexcited charge carriers lose their energy rather quickly through different inelastic and elastic scattering processes over a characteristic distance L, the so-called scattering length. In a HIP device, a potential barrier is formed between the heavily and the lightly doped semiconductor region, parallel to the surface of the HIP device. If a photoexcited charge carrier is produced less than the scattering length L away from the potential barrier and if the energy of the photoexcited charge carrier is sufficiently high, then the charge carrier can overcome the potential barrier, it is transported vertically though the lightly doped region into the semiconductor, where it can be detected with one of the electronic circuits known from literature. Such HIP photosensors, made for example from silicon or germanium, have been used for the detection of infrared radiation with a wavelength exceeding 200 micrometers.
However, HIP infrared photosensors suffer from two main disadvantages: (1) The potential barrier which the excited charges must overcome is permanently fixed by the materials employed for the fabrication of the HIP photosensor; it can be determined by the work function of a particular metal, or it is governed by the doping concentration of the lightly doped semiconductor volume. As a consequence, the cutoff wavelength of such a HIP photosensor cannot be electrically adapted. (2) The incident infrared photons create excited charge carriers that must diffuse vertically through the highly-doped conversion region before reaching the lightly doped semiconductor volume where they can be detected. Since excited majority charge carriers in highly doped semiconductors have very short lifetimes before thermalization, their diffusion length in the highly doped semiconductor is limited to short distances of the order of nanometers. As a consequence, the effective quantum efficiency of such HIP photosensors is very low compared with photosensors exploiting the photoelectric effect in depleted semiconductor regions.